Power line communication systems have found relatively little application in the United States. Nevertheless, power lines would appear to present an ideal, preexisting communication medium for a network in which the expense of the usual forms of network media are unwarranted, such as a home network. For example, one could visualize that, in the not too distance future, all appliances and all electrical and the electronic devices in a home could be connected in a network through their power lines, so that the homeowner could control all such devices through his computer or appropriate terminal and have ready access to such services as entertainment, Internet and telephone.
Despite the attractiveness of a power line communication network, it does present its own unique problems. For one thing, the electronic, electrical and electromechanical devices connected to the power lines of a home produce various types of electrical noise which finds its way onto the power lines and make them a relatively noisy medium. This not only means that appropriate measures need to be taken to assure reliable synchronization and acceptable bit error rates, but collision detection becomes very difficult necessitating a token passing protocol for reliable media access control.
The token ring protocol relies on a physical ring topology which is inapplicable to the power line network, where all stations are essentially on one line or bus. A power line network is more amenable to the Token Bus Protocol. However, that protocol requires that all the stations on the medium must be able to hear all other stations so that they can be given access to the medium in a logical ring arrangement. In a power line local area network, some of the stations may not be able to hear others or may be out of communication range. In accordance with the present invention, it therefore became necessary to define a new protocol for the Media Access Control (MAC), which will be referred to herein as the Powerline MAC (PMAC) protocol.
In accordance with the present invention, there is also defined a unique network architecture. It includes a virtual LAN (vLAN), which contains all the stations constituting a single customer's network, such as a home network. A vLAN could, for example, include all of the stations in a single home or a single apartment within a building.
Since power lines are shared among different homes or different apartments, it is likely that different vLANs will interfere with each other on the power line. The network architecture therefore includes a group LAN (gLAN), which contains a plurality of vLANs, each of which interferes with at least one other vLAN. The vLANs comprising a gLAN share a single token, to ensure that two stations on such vLANs cannot talk at the same time.
The system architecture also contemplates a PMAC, which comprises the entire power line on the customer side of the last step-down transformer that delivers power to the customers.
In accordance with the PMAC protocol, a two-level token passing loop is provided. On the first level, there are the stations comprising a vLAN, and inside the vLAN, a vLAN coordinator acts as a vLAN controller to manage access to the network by its nodes, using polling. On the second level, the vLAN coordinators from each vLAN act as gLAN controllers to form a gLAN token passing logical ring by using a double-linked loop in which two vLANs which cannot communicate to each other pass the token through a third vLAN with which both vLANs can communicate. The third vLAN merely passes the token between the other two vLANs, without retaining it. The third vLAN will, however, be able to retain the token when it receives it in its normal sequence in the logical ring. Thus, each vLAN coordinator receives the token once during its travel around the logical ring as a potential token holder, but it may receive a token multiple times as a token passer.
In accordance with another aspect of the present invention, the PMAC protocol includes a three-way handshake process for token passing. A completed token pass involves the transfer of a “token pass” message from the originator to the recipient, an acknowledgment (ACK) by the recipient, and a handshake return by the originator. In order to secure against the loss of token in the noisy power line environment, the three-way handshake sends a retry remaining count with the token pass and waits for an ACK response. If the ACK is received, a handshake is sent, but if it is not, the token pass is resent with a decremented retry remaining count until the count is zero. On the recipient side, upon receiving a token pass, an ACK is sent and the retry remaining count is decremented. If a handshake is received, the recipient takes control. Otherwise, the recipient decrements the retry remaining count and resends the ACK and retry remaining count, until a retry remaining count is zero.
In accordance with a further aspect of the PMAC protocol, a unique process is provided for allocating access time to the transmission medium. A token rotation time (TRT) is defined for each round trip of the token through the logical ring of the gLAN. One of the vLAN coordinators is defined as the Token Master, which determines a vLAN token hold time vTHT for each vLAN by dividing the TRT by the total number of vLANs. Token mastership is transferred after each round trip of the token through the gLAN. Loss of token mastership is prevented by maintaining a token heartbeat which is incremented each time token mastership is passed. As soon as a vLAN coordinator observes the same token heartbeat twice, it seizes mastership to prevent loss of mastership.
In accordance with another aspect of the PMAC protocol, polling overhead is managed by granting medium access to nodes of an active connection at a rate required to maintain quality of service guarantees. A node without an active connection is granted access at a much slower rate determined at the convenience of the protocol.
In accordance with another aspect of the PMAC protocol, a narrow synchronization window is enabled around the expected synchronization preamble to increase the chances of receiving a proper transmission in the noisy power line medium. Precise inter-frame gap timing and predictable response size are provided. The precise inter-frame gap allows the transmitter to trigger its receiver in relationship to the end of its transmission, and the predictable response size allows the transmitter to time the start of its next transmission in the event that it misses the receiver's response.
In accordance with another aspect of the invention, the reliability of synchronization and equalizer adjustment are enhanced by sampling each symbol of a received signal at a multiple (as used herein, this will be understood as not necessarily an integer multiple) of the Nyquist rate for the number of symbols corresponding to the synchronization preamble. Specifically, N samples (where N is a multiple of the Nyquist rate) are acquired for each of the first M symbols in the received signal, where M is the number of symbols in the synchronization preamble. Each group of samples corresponding to a symbol is then correlated with the nominal sequence of symbols comprising the synchronization preamble, and the BER is determined between the nominal sequence and each sample position of the received symbols. The preceding steps are repeated in successive sample positions of the received signal. For synchronization purposes, we use the position of the received signal which yields the lowest BER.
In accordance with another aspect of the invention, an equalizer is permitted to converge on an optimal setting to achieve minimal BER. Modulation with a lower bit rate is used while the equalizer converges and is changed to the normal bit rate as the optimal setting is approached.